This invention relates generally to fracturing subterranean formations and to fracture monitoring methods.
There are various uses for fractures created in subterranean formations. In the oil and gas industry, for example, fractures may be formed in a hydrocarbon-bearing formation to facilitate recovery of oil or gas through a well communicating with the formation.
Fractures can be formed by pumping a fracturing fluid into a well and against a selected surface of a formation intersected by the well. Pumping occurs such that a sufficient hydraulic pressure is applied against the formation to break or separate the earthen material to initiate a fracture in the formation.
A fracture typically has a narrow opening that extends laterally from the well. To prevent such opening from closing too much when the fracturing fluid pressure is relieved, the fracturing fluid typically carries a granular or particulate material, referred to as xe2x80x9csandxe2x80x9d or xe2x80x9cproppant,xe2x80x9d into the opening of the fracture. This material remains in the fracture after the fracturing process is finished. Ideally, the proppant in the fracture holds the separated earthen walls of the formation apart to keep the fracture open and provides flow paths through which hydrocarbons from the formation can flow at increased rates relative to flow rates through the unfractured formation. In another application, acids are used to create uneven surfaces so that the fracture does not completely close, thus still providing effective flow channels through the fracture.
Such a fracturing process is intended to stimulate (that is, enhance) hydrocarbon production from the fractured formation. Unfortunately, this does not always happen because the fracturing process can damage rather than help the formation (for example, proppant can clog the fracture tip to produce a xe2x80x9cscreenoutxe2x80x9d condition).
Stimulating wells that behave nicely (for example, wells that are easily stimulated) allows service companies and operators to follow standard procedures commonly performed on such wells. No special attention needs to be placed upon specifics, such as how the fracture behaves; decisions and actions are based upon the experience the industry has acquired over many years.
However, as the hydrocarbon supply decreases and demand for it increases, the hunt for hydrocarbons becomes more challenging. New technologies, such as fluid chemistry and rheology, or even new stimulation techniques enter the marketplace. These techniques claim to provide better fracture creation, better conductivities, permeability modifications, and more. As these technologies are used, new methods for evaluating the effectiveness of the treatments are needed.
In at least these more challenging situations, fracture behavior is an important aspect in fracturing technology. Many techniques are available for pre-stimulation simulations and post-stimulation analyses of fracture behavior; however, few techniques address fracture behavior during the stimulation process itself. Various fracture behaviors, such as fracture extension, ballooning, and tip screenout are often not known to the operator until after it is too late or even after the job is completed. Therefore, there is a need for real-time analysis or monitoring of fractures.
The present invention meets the aforementioned need by providing a novel and improved fracture monitoring method and fracturing method.
Certain changes occurring downhole during a fracturing process, such as fracture extension, send different pressure frequency spectra and wave intensities to the surface. In accordance with the present invention, these signals can be processed to reveal information about one or more aspects of the downhole environment. That is, capturing and evaluating generated and reflected pressure waves during fracturing enables personnel to monitor, in real time or later, what happens downhole during fracturing.
Any time a fracture extends, there is a sudden burst of acoustic noise embodied in a pressure wave or signal. Noise coming from other sources also contributes to this signal. By converting the time based pressure signal to a frequency base using a Fourier transform, for example, one can monitor this acoustic noise. In a particular implementation of the present invention, this is implemented with a waterfall plot of frequency spectra at successive time slices of the original signal. In such a waterfall plot, and in accordance with the present invention, a ridge of decreasing frequencies indicates fracture extension and a ridge of increasing frequencies indicates either closure or sand/proppant backing up in the fracture. By summing the area under the spectral plot, one can also get an indication of the energy drop as the fracture extends and sudden rise at a screen out.
A fracture monitoring method in accordance with the present invention comprises: creating frequency spectrum data in response to a pressure in a well sensed over time during a fracturing process performed on the well; and determining from the frequency spectrum data at least one characteristic of a fracture formed by the fracturing process. This can include one or more of the following, for example: determining, in response to a declining frequency defined in the frequency spectrum data, that the fracture is being extended by the fracturing process; determining, in response to an increasing frequency defined in the frequency spectrum data, that the fracture is effectively not being extended by the fracturing process; and determining, in response to an increasing frequency defined in the frequency spectrum, that proppant is backing up in the fracture.
In one embodiment, creating frequency spectrum data includes applying a frequency transform to data of the sensed pressure. Examples of frequency transform include a Fourier Transform in general and a Short Time Fourier Transform in particular.
Creating frequency spectrum data can also include filtering data of the sensed pressure. Such filtering includes wavelet filtering in one embodiment of the present invention.
A fracture monitoring method of the present invention can also be defined as comprising: sensing pressure over time during a fracturing process performed on a well such that pressure data is obtained; making a frequency analysis of the pressure data, including making a waterfall plot of frequency data obtained in response to the pressure data; and using the waterfall plot to determine at least one characteristic of a fracture formed by the fracturing process. Using the waterfall plot in one embodiment of the present invention includes identifying one or both of (1) a declining ridge section for a selected frequency range over a period of time and (2) an increasing ridge section for the selected frequency range.
The present invention can also be defined as a computer-implemented fracture monitoring method, comprising: receiving in a computer pressure data obtained over time from a well undergoing a fracturing process; performing in the computer a transform on pressure data received in the computer to provide frequency data for selected times of the pressure data; and using the frequency data to determine whether a fracture created by the fracturing process is extending, including determining decreasing and increasing frequency sections within the frequency data.
A fracturing method of the present invention broadly comprises: pumping a fracturing fluid into a well such that a fracture in an adjacent formation forms and pressure signals are generated; sensing the pressure signals; determining frequencies at various times of the sensed pressure signals; creating a plot of the frequencies at the various times; and determining from the plot whether the fracture is extending into the formation. This can further comprise controlling further pumping of the fracturing fluid in response to determining whether the fracture is extending.
Other aspects consistent with the foregoing are included in further definitions of the present invention.
Therefore, from the foregoing, it is a general object of the present invention to provide a novel and improved fracture monitoring method and fracturing method. Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art when the following description of the preferred embodiments is read in conjunction with the accompanying drawings.